Think of it this way, imagine going on a road trip and planning to see 30 cities in the USA, and saying that you’ll arrive home at 4:12pm exactly 71 days from now. You’re not allowed to arrive home early, if you do you’ll have to drive to see another city, and if you arrive later, oh boy… just make sure you don’t arrive later.
We need incentives for project people… it’s time
Dawid Naude

And in most cases, the person making the estimate is not the person driving the car.